1. Technical Field
The present invention relates to an underwater carriage for the inspection of the inner surface of a pressure vessel of a nuclear reactor and, more particularly, to the mounting of the transducers on the underwater carriage.
2. Description of the Prior Art
Under the Atomic Energy Act of 1954, the United States Nuclear Regulatory Commission (“NRC”) has the authority to inspect nuclear power plants to protect public health and safety. A part of the NRC inspection program assesses whether the equipment is properly maintained to ensure safe operation.
Reactor inspections are conducted by independent inspectors to provide an assessment of the plant's condition and performance. One of the inspections that is routinely performed on a nuclear power plant is an inspection of the walls of the reactor vessel, reactor containment vessel, and/or core shroud.
There is a family of nuclear reactors known as light water reactors (LWR) which are cooled and modulated with ordinary water and tend to be simpler and cheaper to build than other types of nuclear reactors. LWRs can be sub-divided into three categories of (1) boiling pressure reactors (BPRs), (2) boiling water reactors (BWRs) and (3) super-critical water reactors (SCWRs). An LWR will have a containment vessel and a reactor pressure vessel. Generally, LWRs are divided into a BWR or BPR type of system.
Regardless of the type of nuclear power plant, the containment vessel, reactor vessel and other vessels used in generating nuclear power have to be inspected to make they are still structurally sound. Such inspections have to take place while the vessels are still filled with a fluid such as water, but while that particular unit of a nuclear power plant is not in operation.
In the past, carriages have been developed to move through water while simultaneously attaching itself to the walls of a vessel. A good example is U.S. Pat. No. 5,730,553, which is incorporated by reference, that has a skirt around the underwater carriage with thrust fans for attaching the underwater carriage to the wall of a vessel being inspected. The underwater carriage would then move around on the wall of the vessel being inspected. The underwater carriage may carry transducers for the inspection of walls of the vessel on which the underwater carriage is located.
Such an underwater carriage with the transducers being carried thereon is illustrated in U.S. Pat. No. 5,852,984, which is incorporated by reference. To ensure the transducers which are ultrasonic probes are in good contact with the surface being inspected, the '984 Patent has sweeping units to make sure the surface of the walls of the vessel are clean and smooth. The ultrasonic probe can then be used to detect flaws and cracks in the walls of the vessel including nuclear reactor pressure vessels. However, in these prior designs, it was always a problem of maintaining the transducer or ultrasonic probe a uniform distance from the inside surface of the wall of the vessel. If a uniform distance is not maintained, false readings could be indicated to the operator at the surface, which could be misinterpreted as a flaw in the vessel.
Applicant has discovered a way to overcome these false readings.